Question

ln aqueous solution, the ionization constants for carbonic acid $\left(H_{2}C{O}_3\right)$ are $K_1=4.2\times {10}^{-7}$ and $K_2=4.8\times {10}^{-11}$. Select the correct statement for a saturated 0.034 $M$ solution of the carbonic acid.

• A. The concentration of $C{O}_3^{2-}$ is 0.034 M.
• B. The concentration of $C{O}_3^{2-}$ is greater than that of $HC{O}_3^{-}$.
• C. The concentration of $H^{+}$ and $HC{O}_3^{-}$ are approximately equal.
• D. The concentration of $H^{+}$ is double that of $C{O}_3^{2-}$.

Answer 1

Answer: (C)

The concentration of $H^{+}$ and $HC{O}_3^{-}$ are approximately equal.

$A\to H_{2}C{O}_3\rightleftharpoons H^{+}+HC{O}_3^{-}$, $K_1=4.2\times {10}^{-7}$

$B\to HC{O}_3^{-}\rightleftharpoons H^{+}+C{O}_3^{2-}$, $K_2=4.8\times {10}^{-11}$

As $K_2<<K_1$, all major $[H^{+}{]}_{total}\approx [H^{+}{]}_A$

where,

$[H^{+}{]}_A=H^{+}$ ion concentration produce in first step.

And from 1 equilibrium, $[H^{+}{]}_A\approx [HC{O}_3^{-}]\approx [H^{+}{]}_{total}$

$\left[C{O}_3^{-2}\right]$ is negligible compared to $\left[HC{O}_3^{-}\right]$ or $[H^{+}{]}_{total}$